Cell cycle progression is regulated by a wide variety of external factors, amongst them are growth factors and extracellular matrix factors. During the last decades evidence has been obtained that reactive oxygen species (ROS) may also play an important role in cell cycle progression. ROS may be generated by external and internal factors. In this overview we describe briefly the generation of ROS and their effects on processes that have been demonstrated to play an essential role in cell cycle progression, including such systems as signal transduction cascades, protein ubiquitination and degradation, and the cytoskeleton. These different effects of ROS influence cell cycle progression dependent upon the amount and duration of ROS exposure. Activation of growth factor stimulated signaling cascades by low levels of ROS result in increased cell cycle progression, or, in case of prolonged exposure, to a differentiation like growth arrest. From many studies it seems clear that the cyclin kinase inhibitor protein p21 plays a prominent role, leading to cell cycle arrest at higher but not directly lethal levels of ROS. Dependent upon the nature of p21 induction, the cell cycle arrest may be transient, coupled to repair processes, or permanent. At high concentrations of ROS all of the above processes are activated, in combination with enhanced damage to the building blocks of the cell, leading to apoptosis or even necrosis.